Astronomy says light slows down.

Does the response to a question about gravitational lensing on page 72 of the March issue of "Astronomy" agree with other statements on this forum about the absolute nature of light speed?

The question asked by "Astronomy" reader Tom Schmidt: "I noticed the photo of Einstein's Cross on page 47 in the November 2003 issue shows four images. Why aren't there many more, even a circle of images because the light from the source would be going in all directions?"

Paul Schechter of the Massachusetts Institute of Technology replied (edited to present only the relevant passage):

"........... In the gravitational case, the gravity of the lensing galaxy acts like the index of refraction of air. It slows down the light as it travels past the galaxy. If the quasar (which we take to be pointlike) aligns sufficiently closely with the galaxy, a second image appears. If the quasar aligns very closely, we get four images........"

Does the response to a question about gravitational lensing on page 72 of the March issue of "Astronomy" agree with other statements on this forum about the absolute nature of light speed?

meddyn,

Sorry no-one has answered you yet.

Dr. Einstein addresses this point in Relativity. While SR rests on the constancy of c, GR supersedes this. Dr. Einstein defends himself along these lines:

Dr. Einstein, Relativity: The Special and General Theory, Crown, New York, p. 76:

A curvature of rays of light can only take place when the velocity of propagation of light varies with position. Now we might think that as a consequence of this, the special theory of relativity and with it the whole theory of relativity would be laid in the dust. But in reality this is not the case. We can only conclude that the special theory of relativity cannot claim an unlimited domain of validity; its results hold only so long as we are able to disregard the influences of gravitational fields on the phenomena (e.g. of light).

Dr. Einstein then goes on to dicsuss the relationship between electrostatic principles and electrodynamic principles in order to show that it is quite normal for one valid theoretical construct to be superseded by a more general one. He ends this discussion thus:

Dr. Einstein, Relativity: The Special and General Theory, Crown, New York, p. 77:

No fairer destiny could be allotted to any physical theory, than that it should of itself point out the way to the introduction of a more comprehensive theory, in which it lives on as a limiting case.

So, your question is a valid step in the process of relating SR to GR. Whether you choose to take the next step toward accepting both GR and SR is up to you.